The present invention relates to a sensor with a diaphragm sensor chip.
Acceleration sensors using diaphragm type pressure sensitive sensor chips have been proposed.
A pressure sensor chip used in such acceleration sensors has a relatively thick base and thin diaphragm. Stain gauges are formed on the surface of the diaphragm by thin-film forming technology. The strain gauges form a bridge circuit. The base of the sensor chip is flexibly adhered to a die pad having lead frames with an adhesive. The lead frames are entirely molded over with a mold material. A space exists about the sensor chip, which is die bonded. The space is filled with silicone gel, which seals the chip.
Each material of the sensor has its own property of thermal deformation. Metal used for the sensor chip and the lead frames has a relatively low coefficient of linear expansion. Resin used for the mold member and the adhesive has a greater coefficient of linear expansion than that of the metal. Great differences of thermal deformation among the materials tends to apply stress to the diaphragm when the ambient temperature changes. This deforms the strain gauges and changes the resistance of the gauges. As a result, the sensitivity of the sensor is changed. Accordingly, the temperature characteristics of the sensor deteriorate.
A few measures have been introduced to address the above draw back. A first measure is to adhere a small ceramic substrate on the die pad and to flexibly attach the sensor chip on the ceramic substrate with elastomer. A second measure is to provide a compensation circuit outside the sensor for electrically compensating the sensor detection signals. These measures have decreased the ratio of change of sensitivity from .+-.10 to 30% to .+-.5%, which is within the tolerance.
However, the first measure requires a ceramic substrate and therefore increases the material cost: In addition, the first measure requires a manufacturing step for adhering the ceramic substrate and therefore complicates the assembly. This lowers the production efficiency.
The second measure requires a compensation circuit and thus increases the cost.
Accordingly, it is an objective of the present invention to provide a sensor having a diaphragm sensor chip that is inexpensive and easy to manufacture, and the sensitivity of which is not greatly affected by temperature.